This invention relates generally to chimney caps and more particularly to a flue cover which acts to tightly close the top of a chimney flue but which can be opened to accommodate egress of smoke.
U.S. Pat. No. 4,528,897 to Homolik discloses a chimney cap which is mounted on top of the chimney flue and can be opened and closed at the fireplace by manipulating a cable which extends through the flue. Chimney caps of this type are more effective in preventing the leakage of air than conventional fireplace dampers because they are able to seal more tightly against the flue. A cover mounted at the top of the flue is also able to prevent rain, snow, insects, birds, small animals, leaves, dirt and other materials from entering the flue. When a fire is burning in the fireplace, the flue cover is opened and can be adjusted to allow the fire to burn at the desired rate. When there is no fire burning, the flue cover should be closed to prevent inside air from escaping and to prevent undesirable materials from entering the chimney.
Despite these benefits, the chimney cap shown in the Homolik patent suffers from a number of problems that have detracted from its commercial success. Perhaps most notably, the spring which opens the device is a simple tension spring which is placed under tension during the great majority of the time when the flue cover is closed. Consequently, the spring is particularly susceptible to being permanently set such that it loses its spring force. Once this occurs, the spring cannot exert enough force on the cover to open properly when the cable is released. The problem of permanent spring set requires frequent replacement of the spring, and this in turn requires work on the roof which can be a dangerous situation. If the spring has set or is not functioning properly for another reason, the user may release the cable and assume that the cover has opened when in fact the spring is not able to open it. If a fire is then started with the cover remaining closed, obvious problems result from the lack of an escape route for the smoke and flue gases.
Another serious problem with the Homolik chimney cap is that it is difficult for many homeowners to install. Mounting of the device requires that the flue tile be drilled so that bolts can be extended through the drill holes to secure the mounting plate in place on the flue. This drilling operation is difficult to carry out accurately and results in permanent holes through the flue tile. Moreover, the holes can weaken the flue tile and in some cases cause it to crumble, crack, or otherwise fail.
The Homolik device is also constructed in a relatively weak manner, especially in the area of the hinge. The cover is hinged to the mounting plate by small tabs which are welded to the plate and thus subject to breaking or bending due both to their small size and to the fact that the welds may fail. The hinge linkage is also welded to the flue cover and this connection is thus particularly susceptible to failure. These structural problems are aggravated by the extreme temperature variations to which the chimney cover is subjected. It is heated to a high temperature in the summer and also in the winter when a fire is burning in the fireplace. Snow, ice and extreme cold create problems in the winter. Due to these temperature fluctuations, the parts can warp and become brittle as a result of thermally induced expansion and contraction, and the device often fails completely or at least loses its ability to provide an air tight seal against the top of the flue.